Phosphorus Sorption in Drainage Channel-Floodplain Systems in Agricultural Watersheds

The two-stage channel is an in-stream conservation practice that creates inset floodplains in agricultural drainage channels and stabilizes streambanks. Inset floodplains are excavated adjacent to the channel, hydrologically reconnecting it to the main channel and tripling the reactive surface area during high flow events. It is uncertain what effect these channel designs have on complex phosphorus (P) dynamics in the stream and in the floodplain, both during baseflow and when channels are flooded. In order to evaluate these practices as a P management strategy, we took soil and water samples from streambed sediments and floodplain soils in 10 two-stage channels in central and northern Indiana during the summer of 2017. Soils were analyzed for their water-extractable P; oxalate-extractable P, Fe, Al; equilibrium P concentration (EPC 0) at zero net sorption; and a P sorption index. We also conducted a particle size analysis, determined moisture content of soils, and calculated loss on ignition to estimate organic matter content. We found that floodplain soils had more water-extractable P and a higher EPC0 than streambed sediments, likely reflective of the elevated SRP concentrations experienced by floodplain soils during storm events. Floodplains also had greater P retention capacity, which was driven by organic matter and amorphous iron oxides providing sites for P binding. Differences between the two-stage channel design and the trapezoidal design were more modest. Streambed sediments were generally similar in the two systems, but inset floodplains had a greater ability to capture P in amorphous iron oxides than naturalized floodplains. As organic matter was the key driver of P retention capacity, differences recovery after the disturbance of construction may have masked differences. Overall, these results indicate that there are relatively minor changes in soil P dynamics after floodplain construction; however, the larger active surface area of the restored floodplains during storm events suggests that P buffering capacity may increase compared traditional trapezoidal channels